In fuel cells employing liquid fuel, such as methanol, and an oxygen-containing oxidant, such as air or pure oxygen, the methanol is oxidized at an anode catalyst layer to produce protons and carbon dioxide. At a cathode catalyst layer, oxygen reacts with the protons to form water. The anode and cathode reactions in this type of direct methanol fuel cell are shown in the following equations:Anode reaction (fuel side): CH3OH+H2O→6H++CO2+6e−  ICathode reaction (air side): 3/2 O2+6H++6e−→3H2O  IINet: CH3OH+3/2 O2→2H2O+CO2  III
The two electrodes are connected within the fuel cell by an electrolyte to transmit protons from the anode to the cathode. The electrolyte can be an acidic or an alkaline solution, or a solid polymer ion-exchange membrane characterized by a high ionic conductivity. The solid polymer electrolyte is often referred to as a proton exchange membrane (PEM). PEMs such as Nafion™ are widely used in low temperature fuel cells, because of the electrolyte membrane's high proton conductivity and excellent chemical and mechanical stability. Since the electrolyte membrane is a polymer with a hydrophobic backbone and highly acidic side branches, the membrane must contain significant amounts of water to conduct protons from the electrode reactions. Therefore, the polymer electrolyte membrane is usually kept in high humidity environment to maintain a high proton conductivity.
PEM fuel cells use basically the same catalyst for both anode and cathode. In addition to undergoing electro-oxidation at the anode, a water soluble liquid fuel, such as methanol, may permeate through the PEM and combines with oxygen on the surface of the cathode electrocatalyst. This process is described by equation III for the example of methanol. This phenomenon is termed “fuel crossover”. Fuel crossover lowers the operating potential of the oxygen electrode and results in consumption of fuel without producing useful electrical energy. In general, fuel crossover is a parasitic reaction which lowers efficiency, reduces performance and generates heat in the fuel cell. It is therefore desirable to minimize the rate of fuel crossover.
There are a number of approaches to reduce fuel crossover. The rate of crossover is proportional to the permeability of the fuel through the solid electrolyte membrane and increases with increasing fuel concentration and temperature. By choosing a PEM with low water content, the permeability of the membrane to the liquid fuel can be reduced. The reduced permeability for the fuel results in a lower crossover rate. Also, fuels having a large molecular size have a smaller diffusion coefficient than fuels having small molecular size. Hence, permeability can be reduced by choosing a fuel having a large molecular size. While water soluble fuels are desirable, fuels with moderate solubility exhibit lowered permeability. In addition, the permeability for vapors is higher than liquids, since fuels with high boiling points do not vaporize and their transport through the membrane is in the liquid phase, fuels with high boiling points generally have a low crossover rate. Furthermore, the wettability of the anode may be controlled by an optimum distribution of hydrophobic and hydrophilic sites, so that the anode structure may be adequately wetted by the liquid fuel to sustain electrochemical reaction, while excessive amounts of fuel are prevented from having access to the membrane electrolyte. Finally, the concentration of the liquid fuel can also be lowered to reduce the crossover rate.
In methanol fuel cells, fuel crossover is typically controlled by using diluted methanol fuel that contains 3% methanol and 97% water by weight. Because the reaction rate is proportional to the reactant, the low fuel concentration results in a low proton generation rate, which in turn leads to limited current drivability and voltage for a given current. Moreover, the fuel concentration gets lower and lower as the methanol is consumed and so does the power. Another problem is fuel efficiency. Since one water molecule (MW=18) is consumed with each methanol molecule (MW=34) in the electrochemical reaction, only about 1.6 wt % water will be consumed with methanol in a fuel composition containing only 3 wt % methanol, the other 95 wt % of water becomes “dead weight”. Therefore, the real “consumable fuel” in the diluted methanol fuel accounts to less than 5% of the total fuel composition.
Other approaches to prevent fuel crossover in fuel cells have been developed. WO 96/29752 to Grot et al. discloses the incorporation of various inorganic fillers into cation exchange membranes made from polymers to decrease fuel crossover. U.S. Pat. No. 5,631,099 to Hockaday discloses fuel cell electrodes having thin films of catalyst and metal materials deposited on fiber reinforced porous membranes. It is suggested that the thin film electrode structure provides the capability to filter the reactant streams of various species, such as carbon monoxide or methanol if the metal electrode materials have selective permeability to hydrogen. U.S. Pat. No. 6,248,469 to Formato et al. discloses composite solid polymer electrolyte membranes which include a porous polymer substrate interpenetrated with an ion-conducting material. Fuel crossover resistance of the membranes can be optimized by using the proper blend of different polymers. None of these approaches, however, has provide satisfactory results.
U.S. Pat. No. 5,759,712 to Hockaday describes a hydrogen-only permeable electrode to block fuel crossover. The invention, however, requires an elaborated membrane structure that contains three layers of metal deposited on a porous membrane.
The major problem with a metal-coated polymer electrolyte membrane is the cracking of the metal coating during hydration when the polymer electrolyte membrane that the metal film covers expands in volume. As demonstrated in FIG. 1A, when a polymer electrolyte membrane 101 covered with a thin metal film 103 is placed in a high humidity environment, the polymer electrolyte membrane 101 absorbs the water and expands in volume. The volume expansion leads to an enlarged surface area and creates very high stress in the thin metal film 103, which eventually results in cracks 105 in the thin metal film 103. Fuel molecules can then permeate the polymer electrolyte membrane 101 through the cracks 105.
It has been found that the expansion-induced cracking of the metal film 103 can be avoided by creating a microtextured surface 107 on the polymer electrolyte membrane 101. As shown in FIG. 1B, the microtextured surface 107 contains many protrusions 108 that flatten out when the polymer electrolyte membrane 101 expands in water. During the flattening process, the thin metal film 103 covering the microtextured surface 107 relieves the expansion-induced stress by rotating towards a center plane 121 of the polymer electrolyte membrane 101, while maintaining the continuity of the metal film 103.
FIG. 2 depicts a typical surface microstructure 200, which comprises a plurality of pyramidal protrusions 201. The protrusions 201 can also be in other shapes, so long as most of the surfaces on the protrusions 201 form an angle with the central plane 121 of the polymer electrolyte membranes 101 and there is minimal flat surfaces between the protrusions 201. The fabrication of surface microstructure 200, however, requires a special mold that is produced by a complicated etching process. Therefore, there remains a need for fuel-impermeable electrolyte membranes that are easily manufactuable.